Oil and natural gas are produced from wells having porous and permeable subterranean formations. The porosity of the formation permits the formation to store oil and gas, and the permeability of the formation permits the oil or gas fluid to move through the formation. Permeability of the formation is essential to permit oil and gas to flow to a location where it can be pumped or flowed from the well. Oil and gas recovery is normally practiced in wellbores which have been cased in steel pipe surrounded by a cementitious fill to seal the wellbore. The artisan will appreciate that the term “casing” is colloquially used to describe a collection of pipe segments that extends from a target depth to the surface of the well; however, in this invention, the term “casing” will be used to describe a pipe segment and the term “casing string” to refer to a collection of pipe segments, which may or may not extend to the surface without any implication of limitation. In many cases the permeability of the formation holding the oil or gas is insufficient for economic recovery of oil and gas. In other cases, during operation of the well, the productivity of the formation drops to the extent that further recovery becomes uneconomical. In such cases, it is necessary to hydraulically fracture the formation and prop the fracture in an open condition by means of a proppant material or propping agent. Such fracturing is usually accomplished by hydraulic pressure through perforations in the steel casing, and the proppant material or propping agent is a particulate material, such as sand, resin coated sand or ceramic particles (all of which can be referred to as “proppant”), which are carried into the fracture by means of a fracturing fluid, sometimes containing high molecular weight polymers, such as guar gum, guar gum derivatives such as hydroxypropyl guar (HPG), carboxymethyl HPG (CMHPG), cellulose, cellulose derivatives such as hydroxyethyl cellulose (HEC), biopolymers, such as xanthan gum and polyvinyl alcohol, which increase the viscosity of the fracturing fluid.
A systematic method of characterizing the location of proppant as placed in a hydraulic fracture at distances from the cased wellbore exceeding more than several inches is currently unavailable. A primary limitation is a method for rendering images of a fracture field based on transmitted and reflected radar signals. A second limitation is the lack of controlled projection of interrogation signals into the formation from antenna assemblies deployed through a metal cased wellbore, receipt of the signals, and methods for analyzing the received signals to form a representative image of the proppant filled fracture.
Ground penetrating radar (“GPR”) systems have been used in uncased wells to characterize objects and features in geologic formations such as natural fractures and salt-domes. Such commercially-available GPR systems are designed to work in uncased wells, and use large discrete antennas.
Existing systems have not been entirely satisfactory in all respects. Thus, there remains a need for improved systems and methods for characterizing objects and features in geologic formations.